Per discussion on -hackers, the attached patch adds the option
-f, --file=FILENAME

to pg_dumpall. In order to support this, a new (undocumented) format
option (-Fa) is added to pg_dump which is identical to -Fp, except that
the output file is opened for append rather than write.
This patch should be applied over the top of my previous patch
(pg_dumpall_default_database3.diff).
Regards, Dave